In a variety of applications, geographic features can be represented by coordinate sets and stored in computer data bases in the form of bits representing the coordinate information. The coordinate information often requires a large amount of storage space on the medium that is used to store it. This always presents a significant problem and becomes particularly undesirable when the storage medium has limited capacity that may not be sufficient to handle the data that must be stored.
Polyline and polygon features can be represented by a starting x-y coordinate pair and a list of coordinate changes between successive coordinate pairs. Geographic features vary widely in size and shape, and some features have large changes in the coordinates while others have only small changes. It is not uncommon, especially with man-made structures, for a feature to have large ranging changes in one coordinate direction and only small changes in the other direction. For example, roadways and grids of streets often follow north-south or east-west paths. Other features, both natural and man-made, exhibit similar characteristics.